1. Technical Field
The present invention relates to a polarization conversion unit, a polarization conversion device equipped with the polarization conversion unit, and a projector equipped with the polarization conversion device.
2. Related Art
In the past, polarization conversion devices for converting an emitted light beam into one kind of linearly polarized light beam have been used in projectors or the like for the purpose of improving the efficiency of the light beam emitted from a light source. Further, the polarization conversion device is composed of a polarization conversion unit, and the polarization conversion unit is composed of a polarization splitting element array composed including a plurality of polarization splitting films disposed obliquely to an incident light beam and for splitting the incident light beam into two kinds of linearly polarized light beams, a plurality of reflecting films disposed in parallel alternately between the polarization splitting films and for reflecting either one of the linearly polarized light beams obtained by the splitting operation of the polarization splitting films, and a translucent member provided with the polarization splitting films and the reflecting films, and a plurality of retardation plates for converting the polarization axis of either one of the linearly polarized light beams obtained by the splitting operation of the polarization splitting films into the polarization axis of the other of the linearly polarized light beams. Further, quartz is sometimes used as the retardation plates in order for enhancing the heat radiation property and so on of the retardation plates.
JP-A-2003-302523 (Patent Document 1) discloses a polarization conversion element (a polarization conversion unit), which uses a retardation plate composed of layered quartz plates, and is formed by bonding the retardation plate to the light beam exit side of the transparent member forming the polarization splitting element array using an ultra violet-curing adhesive.
However, as described in Document 1, in the case in which the retardation plate is bonded on the light beam exit side of the transparent member using the ultra violet-curing adhesive, it is difficult to adhere the light beam entrance side surface of the retardation plate and the light beam exit side surface of the transparent member (the translucent member) to each other since the surfaces of the transparent member and the retardation plate are not perfect planes, and there is caused a microscopic clearance no greater than the wavelength of the light beam between the light beam entrance side surface of the retardation plate and the light beam exit side surface of the transparent member (the translucent member). Therefore, damage to the optical characteristic such that an optical defect (such as an interference pattern) is easily caused in the projected image can be cited as a problem. Further, it can also be cited as a problem that, in the case in which the retardation plate coated with the ultra violet-curing adhesive is mounted on the light beam exit side surface of the transparent member in the manufacturing process for bonding the retardation plate with the light beam exit side of the transparent member, the retardation plate can hardly be remounted in that condition if the mounting position of the retardation plate is not appropriate for some reasons, which causes increase in the manufacturing cost.
It should be noted that in the case of remounting the retardation plate to the appropriate position, it is required to reuse the retardation plate by cleansing away the applied ultra violet-curing adhesive, or to use another retardation plate coated with the ultra violet-curing adhesive. In addition, the same treatment is required to the light beam exit side surface of the transparent member because the ultra violet-curing adhesive is also attached to that surface. At any rate, this treatment also causes increase in the manufacturing cost.